As described in Patent Document 1, a known pressure sensor includes a sensor chip having a depressed portion and a diaphragm equipped with a gauge resistor, a support member defining a pressure transmission passage communicating with the depressed portion, and a gel member integrally filling the depressed portion and the pressure transmission passage. In the pressure sensor, the diaphragm is deformed according to pressure transmitted through the gel member.
This kind of pressure sensor may be used to measure a difference in pressure before and after an exhaust gas purification filer (e.g., diesel particulate filter abbreviated as DPF) installed in an exhaust pipe of a diesel engine vehicle. Alternatively, the pressure sensor may be used in an exhaust gas recirculation (EGR) system to measure pressure. In order to protect the pressure sensor against a corrosive pressure medium such as liquid and gas, the pressure sensor is provided with the gel member. In such a pressure sensor, if water enters the depressed portion of the sensor chip, the freezing and volume expansion of the water at lower temperatures may damage the diaphragm. Because of this, the gel member is provided to prevent the water from entering the depressed portion through the pressure transmission passage of the support member.    Patent Document 1: JP-2007-3449A (US 2006/0288793A)
In the pressure sensor, the support member (e.g., stem) made of glass may be bonded to the sensor chip by anodic bonding. In general, the anodic bonding is performed under a condition that the stem is grounded to have a low electric potential and the sensor chip has a high electric potential. In this case, when the sensor chip has a shoulder portion that projects radially inwardly than the pressure transmission passage, the shoulder portion acts as a lightning rod at the anodic bonding, and as a result, a current flows from the shoulder portion. That is, a spark occurs. This reduces a bonding strength and makes a stable anodic bonding difficult. In the above, the sensor chip has the shoulder portion when the open end of the depressed portion of the sensor chip on a support member side is smaller than an open end of the pressure transmission passage of the support member on a sensor chip side.
Patent Document 1 illustrates a pressure sensor in which a diameter of an open end of the pressure transmission passage of the stem on a sensor chip side is smaller than the diameter of the open end of the depressed portion of the sensor chip. The stem has a shoulder portion that radially inwardly projects than the outer edge of the depressed portion of the sensor chip. When the stem has the shoulder portion, the spark may be prevented from occurring.
At low temperatures of, for example, −30 degrees C. or less, the gel member used in the pressure sensor hardens and moves (flows) to relax a resultant stress. As a result, a resistance of the gauge resistor of the diaphragm changes and a sensor output property changes. At high temperatures, the gel member may expand and the sensor output property may accordingly change. When the pressure sensor is placed in an exhaust gas environment and exposed to an acid component of the exhaust gas such as nitric acid and the like for a long period of time, a surface layer of the gel member is hardened. When the surface layer of the gel member is hardened, the sensor output changes to a large extent. In particular, at high temperatures, the sensor output changes to a large extent due to the hardening of the gel member.
As described above, the stem may have the shoulder portion. In this case, when the gel member moves from the depressed portion toward the stem, a viscous resistance of the gel member against the stem is large. This viscous resistance inhibits the gel member from moving toward the stem. When the gel member moves from the stem toward the depressed portion, a viscous resistance is small. Thus, when the stem has the shoulder portion, the gel member can more easily move from the stem to the depressed portion than when the sensor chip has the shoulder portion. The stress may acts on the diaphragm.